In pipelines for transport of natural gas and crude oil over long distances, a reduction in transportation cost has been a universal need, and efforts have focused on improvement of transport efficiency by increasing the maximum working pressure of the pipeline. The standard approach to increasing maximum working pressure involves increasing the wall thickness of commercially available linepipe. However, due to the increase in steel tonnage, this approach results in higher material costs, higher transportation costs, higher on-site welding costs, and a reduction in overall pipeline construction efficiency. An alternate approach is to limit the increase in wall thickness by enhancement of the strength of the linepipe material itself. For example, the American Petroleum Institute (API) recently standardized X-80 grade steel. “X-80” means a yield strength (YS) of at least 551 MPa (80 ksi). More recently, even higher strength steels suitable for use in pipelines have been developed that provide pipe with a yield strength of at least 620 MPa (90 ksi) and as high as about 965 MPa (140 ksi), but these steels have not yet been applied commercially. These new higher strength steels suitable for pipelines are made by the Thermo-Mechanical Controlled Rolling Process (TMCP), which imparts much of the strength and toughness by controlled rolling of the plate within specified temperature ranges followed by accelerated cooling, thus achieving a specific microstructure and grain size.
When a pipeline is constructed there is a need for non-regular shaped pieces of pipe called fittings. These pieces, when welded into the pipeline, enable a change in the pipeline direction (elbows or bends); joining of pipes of different diameters (reducers or expanders); or splitting a pipeline to permit flow in or out from two directions (Y and T shaped junctions). To ensure that the integrity of the pipeline is maintained, these special pieces must have the same burst capacity as the pipe used to make the pipeline.
At the present time, fittings with yield strengths of up to about 65 ksi to 70 ksi are available commercially. Further, there has been at least one case where X-80 fittings were made on a special order. For pipeline grades above X-70 (YS=70 ksi), commercially available fittings of comparable strength do not exist. Therefore, the approach presently used for higher strength pipelines (e.g., X-80 pipelines) is to use fittings of lower strength but make them with a wall thickness greater than that of the linepipe such that the burst capacity is maintained. The relationship between the wall thickness and burst capacity is shown below as equation 1:
                              T          w                =                                            P              b                        ×            D                                2            ×            UTS                                              (        1        )            
Wherein Tw is the wall thickness of the pipeline (pipe or fitting), Pb is the burst pressure of the pipeline, D is the outside diameter of the pipeline, and UTS is the ultimate tensile strength of the pipeline material. In a pipeline, pressure and diameter are essentially constant. Therefore, the wall thickness of the fitting, relative to the pipe wall thickness, must essentially be equal to the ratio of the ultimate tensile strengths as shown in equation 2:TFitting=(Tpipe)×(UTSPipe)/(UTSFitting)  (2)
Wherein TFitting is the thickness of the fitting, TPipe is the thickness of the steel linepipe, and UTS is the ultimate tensile strength of the respective material. There are some constraints to this approach, including codes restricting the amount of wall thickness mismatch between the pipe and fitting to a ratio of 1.5. This is done to minimize localized straining. Since X-70 is the highest strength fitting made on a commercial basis, pipes with strength above about X-100 cannot be welded directly to an X-70 fitting.
Thus the industry has two choices for pipelines using linepipe with a strength greater than X-100. One choice is to develop new, higher strength fittings which eliminates the wall thickness mismatch issue. The second choice is to use thicker wall fittings in combination with thick wall transition pieces to minimize the wall mismatch at each joint. While the second choice is feasible, it is not the most effective approach.
Many commercially available high strength steels are limited in their use, compared to lower strength steels, particularly in fracture critical applications, because they typically have lower fracture toughness (thus, limited defect tolerance). Pipes and fittings must have adequate fracture toughness. Toughness in steel may be evaluated by several different methods or criteria (e.g., the ductile-to-brittle transition temperature (DBTT) measured by the Charpy V-Notch (CVN) test, the magnitude of the absorbed CVN energy at a specific temperature, or the magnitude of the fracture toughness at a specific temperature as measured by a test like the crack tip opening displacement (CTOD) test or the J-integral test). All of these above referenced toughness testing techniques are known to those skilled in the art (See Glossary for definition of DBTT and CTOD).
In addition, there is a need for the steel to be weldable (i.e., the weldment is not susceptible to hydrogen cracking when conventional arc welding techniques such as gas metal arc welding and shielded metal arc welding techniques are used to produce the weldment and when preheating is limited to less than about 150° C.). To provide a weldable, hot formed high strength steel component, the total alloying content in the starting high strength steel of the present invention is preferably limited to a Pcm of less than or equal to 0.35 (See Glossary for definition of Pcm). Accordingly, there is a need for higher strength fittings and other components that have adequate fracture toughness and that can be formed from weldable steel. The present invention satisfies this need.